The proteins subjected to digestion and absorption are obtained from two sources.

1.Exogenous2.Endogenous

The Fate of Dietary ProteinThe intake of dietary protein is in the range of

50-100g/day.Digestion and absorption .Maintenance of body protein stores.Net protein synthesis. Synthesis of non-protein compoundsOxidative deamination

PROTEINS in the BODYAmino Acid Pool – amino acids that are

available throughout the body (tissues and fluids) for use when needed.

Protein Turnover – of the ~ 300 grams of protein synthesized by the body each day, 200 grams are made from recycled amino acids.

Protein DigestionWhole proteins are not absorbed.

Too large to pass through cell membranes intact.

Digestive enzymes.HydrolasesBreak peptide bonds

Secreted as inactive pre-enzymes.Prevents self-digestion.

H3N+ C

HC

R

O

NH

CH

CO

RNH

CH

C

R

O

O–

Protein DigestionInitiated in stomach

HCl from parietal cellsStomach pH 1.6 to 3.2Denatures 40, 30, and 20 structures

Pepsinogen from chief cells

Cleaves only when carbonyl group of the peptide bond is contributed by Aromatic amino acids.

Protein leaves stomach as mix of insoluble protein, soluble protein, peptides and amino acids

Pepsinogen HCl Pepsin

Protein Digestion – Small IntestinePancreatic enzymes secreted

TrypsinogenChymotrypsinogenProcarboxypeptidaseProelastaseCollagenase

Zymogens

The release of pancreatic zymogens is mediated by the secreation of Cholecystokinin and secretin,two polypeptide hormones of digestive tract.

Digestion inSmall Intestine

Zymogens must be converted to active formTrypsinogen Trypsin

EndopeptidaseCleaves on carbonyl side of Lys & Arg

Chymotrypsinogen ChymotrypsinEndopeptidase

Cleaves carboxy terminal Phe, Tyr and TrpProcarboxypeptidase Carboxypeptidase

ExopeptidaseRemoves carboxy terminal residues

Enteropeptidase/Trypsin

Trypsin

Trypsin

Trypsin InhibitorsSmall proteins or peptidesPresent in plants, organs, and fluids

Soybeans, peas, beans, wheat Pancreas, colostrum

Block digestion of specific proteinsInactivated by heat

Protein DigestionProteins are broken down to

TripeptidesDipeptidesFree amino acids

Peptide Absorption

Form in which the majority of protein is absorbed

More rapid than absorption of free amino acids

Active transportEnergy required

Metabolized into free amino acids in enterocyte

Only free amino acids absorbed into blood

Free Amino Acid AbsorptionFree amino acids

Carrier systems Neutral AA Basic AA Acidic AA Imino acids

Entrance of some AA is via active transport Requires energy

Na+ Na+

Protein DigestionSmall intestine (brush border)

Aminopeptidases Cleave at N-terminal AA

Dipeptidases Cleave dipeptides into Aas.

(Enterokinase or enteropeptidase) Trypsinogen trypsin Trypsin then activates all the other enzymes

In the Enterocytes…First cells that can use

the amino acids Transport into portal

bloodProtein synthesis

Digestive enzymesStructure and

growthEnergy

Groff & Gropper, 2000

*Whole proteins are nutritionally insignificant...

Basolateral MembraneTransport of

free amino acids only*Peptides are

hydrolyzed within the enterocyte

Transport mainly by diffusion and Na-independent carriers

Absorption of Intact ProteinsNewborns

First 24 hours after birthImmunoglobulins

Passive immunityAdults

Paracellular routesTight junctions between cells

Intracellular routesEndocytosisPinocytosis

Of little nutritional significance...Affects health (allergies and passive immunity)

Abnormalities in the protein digestion and amino acid absorption.Defect in the pancreatic secreation.

Cystic fibrosis,incomplete digestion of fat and protein,results in abnormal appearance of lipids (steatorrhea) and proteins in feces.

Defective carrier system

HARTNUP’S DISEASEInability of itestinal and epithelial cells

to absorb neutral amino acids.Tryptophan absorption is severely effected resulting in pellagra.

CYSTINUREA.